This invention relates to cryogenic cooling apparatus of the type including a generally tubular heat exchanger affording two paths, through one of which, in use, refrigerant gas from a supply under pressure is supplied to a Joule-Thomson expansion nozzle to liquefy a portion of the gas in a container, whence the low pressure gas returns through the other path, and a valve member co-operating with the nozzle to vary its effective area for automatically controlling the flow of the refrigerant, the valve member being actuated via an elongate operating member by a movable wall, preferably of a bellows, situated within or beyond the heat exchanger, and exposed on one side to the pressure of a sensing vapour which, in operation, is in equilibrium with liquid.
Unlike ordinary thermostatic cooling apparatus, apparatus of the type referred to, in which gaseous refrigerant is liquefied, cannot be controlled by the temperature of the refrigerant since this remains constant so long as refrigerant liquid is in equilibrium with refrigerant vapour at constant pressure. Thus the aim is to control the cooling in accordance with the amount of refrigerant liquid present in the container, either in a pool or in the form of a spray of droplets.
Thus some constructions of this type, as exemplified by those in British patent specification Nos. 1368107 and 1297133, have hitherto been provided with a stationary sensor in the form of a tube communicating with the sensing vapour space, and extending from it down to the region of the valve. Such an arrangement provides a sensitive response to the amount of liquid in contact with the sensor, whether in the form of a pool in the container or in the form of a spray or droplets from the nozzle, but its accomodation in the available space may prove difficult, particularly in the very small coolers often required.